Negative Feedback

Negative Feedback Definition

Negative feedback is a type of regulation in biological systems in which the end product of a process in turn reduces the stimulus of that same process. Feedback, in general, is a regulatory mechanism present in many biological reactions. By allowing certain pathways to be turned off and on, the body can control various aspects of its internal environment. This is similar to flipping a switch. Feedback allows the product of a pathway to control the switch. Sometimes referred to as a “negative feedback loop”, negative feedback occurs when the product of a pathway turns the biochemical pathway off. Positive feedback, the opposite of negative feedback, is found in other biological pathways in which the product increases the pathway. Below are examples of negative feedback.

Examples of Negative Feedback

Regulating Blood Sugar

Every time you eat, a negative feedback mechanism controls the level of sugar in your blood. The main sugar found in your blood is glucose. After you eat something, your body absorbs the glucose from your bloodstream and deposits it into your blood. This increases the concentration of glucose and stimulates you pancreas to release a chemical called insulin. Insulin is a cellular signaling molecule which tells muscle and liver cells to uptake glucose. Liver cells store the excess glucose as glycogen, a chain of glucoses used as a storage product. Muscle cells can store the glucose or use it to make ATP and contract. As this process happens, glucose concentrations are depleted in the blood. Glucose was the main signal for the pancreas to produce insulin. Without it, the pancreas stops producing insulin and the cells stop taking up glucose. Thus, glucose levels are maintained in a specific range and the rest of the body has access to glucose consistently. The negative feedback mechanism in this system is seen specifically in how high glucose levels lead to the pathway turning on, which leads to a product meant to lower the glucose level. When glucose becomes too low, the pathway shuts off.

Temperature Regulation

All endotherms regulate their temperature. Endotherms are animals which regulate their bodies at a different temperature than the environment. You can think of mammals and birds as the most common endotherms. Most of the pathways responsible for temperature regulation are controlled by negative feedback. As the temperature rises, enzymes and pathways in the body are “turned-on”, and control various behaviors like sweating, panting and seeking shade. As the animal does these things, the temperature of their body starts to decrease. The activity of these pathways, which is driven by the heat, also starts to decrease. Eventually, a temperature is reached at which the pathway shuts off. Other pathways are present for temperatures that are too cold, and are also shut off once the body reaches the optimal temperature. These pathways can be shivering, seeking shelter, or burning fat. All these activities heat the body back up and are shut off by the end product of their reactions, heat.

Filling a Toilet Tank

Many students tend to struggle with abstract biological examples of negative feedback. Have no fear! A simple and common house-hold item uses negative feedback every day. In the tank on the back of your toilet is a ball or float, which rests at water level. When you empty the tank, the water level drops. The pressure from the float that was holding the valve shut releases, and new water flows into the tank. The valve controlled by the float is like an enzyme that monitors the level of the product it creates. As more water (product) fills the tank, the float slowly decreases the amount of water being let in through the valve. The valve is analogous to an enzyme which is regulated by feedback from a product it helps create or let into a cell.

Quiz

1. Which of the following represents negative feedback?A. Blood platelets release chemicals that attract more blood platelets when then fill a woundB. One bird fleeing a predator spurs three birds, which in turn scares the whole flockC. In producing an amino acid, the enzyme a cell uses is inhibited after the amino acid reaches a specific concentration

Answer to Question #1

C is correct. The first two systems represent positive feedback. As a few individuals start to react, many more are encouraged to react. These systems result in reactions that go to completion in one direction. For example, the entire flock will fly away or the entire wound will be sealed. In the third case, the product regulates the pathway. This means that the cell will not expend too much energy and will produce just the right amount of the product it needs.

2. Bees control the temperature of their hive in an interesting way. When the temperature gets too hot, certain bees release a signal to the rest of the colony to begin a specific behavior. The bees evaporate water from their mouths and fan their wings to significantly decrease the temperature. As it cools, the colony resumes its normal activities. Which of the following terms describes this scenario?A. Positive FeedbackB. Negative FeedbackC. Enzyme Inhibition

Answer to Question #2

B is correct. This is an example of negative feedback. The stimulus produces a reaction in the bees which lowers the stimulus. In turn, the pathway is eventually shut off. Remember that feedback mechanisms can be a part of systems of all sizes, from chemical pathways to the activities of entire groups of organisms.

3. You are reaching into a hot stove to grab your dinner. Your finger slips off the hot pad, and touches the scalding hot dish in the oven. A signal is sent to your brain, which tells your arm to contract. When your finger stops burning, your arm can relax. What does this scenario represent?A. Negative FeedbackB. Positive FeedbackC. Fight or Flight response

Answer to Question #3

A is correct. Again, the stimulus that caused the reaction is removed through the process. This is negative feedback. The fight or flight response may be involved, but remember that even these processes must be controlled by some form of feedback, or else they would continue forever. The negative feedback mechanism allows the system to reset after a stimulus, which at the cellular level allows for preparation for another stimulus to react to.